344 CHEMISTRY OF THE PROTEIDS CHAP. 



that haemoglobin does not pass through porous earthenware filters. 

 The author explains this phenomenon as due to the same causes as 

 those which lead to the mutual precipitation of two colloidal solutions 

 of opposite sign. 



The firm albuminous compounds, such as fibrin and the tissue- 

 forming elements, react in the same way as do the soluble albumins ; 

 for when coagulated by heat, alcohol, metallic salts, and formaldehyde, 

 they become denaturalised and lose their natural properties. The 

 hardening and fixing of organs for histological purposes by means of 

 the salts of the heavy metals, by acids, aldehydes, alcohols, and 

 osmium tetroxide, depend on * coagulation ' being produced. There 

 is, however, a great difference between the electrolytes, which cause 

 structural changes, and the non-electrolytes, such as osmium tetroxide 

 and formaldehyde, which preserve the minute structure of colloidal 

 substances by forming additive compounds, as is fully discussed in 

 the author's Physiological Histology. 



Properties of Denaturalised Albumin 



As soon as denaturalisation occurs, all albumins lose their specific 

 solubilities, and they resemble one another in their denaturalised state 

 in being insoluble in water and in neutral salt solutions, while they 

 are soluble in alkalies and acids. The acid-albumins and alkali- 

 albuminates resemble one another further in being much more soluble 

 in dilute alcohol than are the native or natural albumins. Other 

 properties, however, such as chemical composition, reactions, salt 

 formation, etc., are not destroyed. As regards histological staining, 

 it is very important that the coagulated tissue-constituents on enter- 

 ing into chemical combination with dye-stuff's are soluble to different 

 extents, and that hydrolysis of these coagulated albumin + dye- 

 compounds takes place quite analogously to that seen with non- 

 coagulated albumin. In what respects coagulated albumin differs 

 from natural albumin, particularly as regards the conversion of pseudo- 

 acids and pseudo-bases into real acids and real bases, is fully discussed 

 in the author's Physiological Histology. The molecular weight of de- 

 naturalised casein is, according to Laqueur and Sackur, 1 not essentially 

 different from that of native casein. 



The * ash-free albumin ' of Harnack 2 has been especially carefully 



1 E. Laqueur and 0. Sackur, Hofmeisters Beitrage, 3. 193 (1902). 



2 E. Harnack, Zeitschr. /. physiol. Chem. 5. 198 (1881) ; also Ber. d. deutsch. 

 chem. Ges. 22. II. 3046 (1889) ; 23. I. 40 (1890) ; 23. II. 3745 (1890) ; 31. II. 

 1938 (1898). 



